kernel - Add debugging and attempt to fix vm_prefault issue

kernel - Add debugging and attempt to fix vm_prefault issue

* Add debugging assertions and attempt to fix a race in the vm_prefault
  code when running through backing_object chains.

* The fix may be incomplete, we really need a way to determine whether any
  chain element has changed state during the scan.  The generation count
  may be too excessive as it also covers vm_page insertions.

Reported-by: Peter Avalos <peter@theshell.com>